The present invention relates to an improved sensor for sensing the level of adhesive in a dispenser for the dispensing of adhesive for a surface mount pick and place machine for mounting electronic components on a printed circuit board. The invention further relates to an improvement in the method of sensing the level of adhesive medium in a dispensing device.
In a surface mount pick and place machine, components are placed accurately and at high speed on a printed circuit board. Known pick and place machines include an adhesive dispensing station, typically mounted adjacent to a placement head; in the case of automatic machines, this adhesive dispensing station can be programmed to provide both single and multiple dot patterns of adhesive.
In both automatic and manual machines where the adhesive used is an epoxy based adhesive, the epoxy is dispensed from a syringe. It is important for the operator to know when the supply of epoxy is exhausted, or about to be exhausted, so that the bonding cycle can be interrupted and the exhausted syringe be replaced or refilled. Known epoxy dispensing syringes are provided with a metal disc which is located in the syringe on top of the reserve of adhesive. As the adhesive is used up, and its level in the syringe falls, the level of the disc falls. A magnetic reed switch is arranged so that it is switched off by the disc as it passes a predetermined position. This arrangement has the disadvantage that is is only a "go/no-go" device, with no analog feedback. This means that the operator has no information as to when the adhesive is about to run out, and no indication of the amount of adhesive remaining in the syringe at a time before the reed switch has been switched, and hence the operator cannot determine how many more bonding cycles can be performed before the syringe has to be replaced or refilled.
It is an object of the present invention to provide a sensor for sensing the level of adhesive medium in a dispensing device in which the above disadvantages are reduced or substantially obviated.
It is a further object of the present invention to provide a method for sensing the level of adhesive medium in a dispensing device in which the above disadvantages are reduced or substantially obviated.